In recent years, clean energy has been demanded for protecting the global environment, such as prevention of harmful influence of CO.sub.2. A solar cell is one of the most expected energy sources because it can utilize the solar energy, which is substantially inexhaustible. A crystalline or amorphous silicon solar cell and a high-efficiency solar cell using a Group III-V compound semiconductor, such as GaAs and InP, have been subjected to practical use. In order to increase the proportion of clean energy in the whole-consumed energy, the consuming electric power should be substituted with a solar cell as much as possible. In order to spread a solar cell to various practical applications, it is important to widen the applicable field, as well as making the device itself to have a high efficiency. From such a point of view, what is receiving attention as an applicable field of a solar cell is, in addition to generation of electricity of relatively large-scale power such as electric power for home, to supply electric power from a solar cell to an apparatus used out of doors, such as a portable electronic apparatus and a portable personal computer.
However, since a large proportion of the surface of such an apparatus is occupied by a display device and an input device, there is no space for mounting a solar cell to the apparatus, and since a solar cell has a certain color, it is difficult to mount a solar cell to the apparatus from the standpoint of design. It is considered to utilize windows occupying a large proportion of an automobile and a building, as a space for mounting a solar cell. However, a solar cell currently available has a certain color and is not transparent since it utilizes a visible ray, and therefore lighting cannot be ensured. Furthermore, it is also considered to utilize other parts than the display device or the windows, but such is currently difficult from the standpoint of design.
The problems can be solved by a transparent solar cell. In other words, the problems can be solved when a high efficiency solar cell for ultraviolet rays can be formed on a transparent substrate.
Investigations of a solar cell have been made to improve the efficiency in a short wavelength region for increasing the sensitivity. However, it has been known that when, for example, the proportion of C in an amorphous Si.sub.1-x C.sub.x film is increased, the photoconductivity characteristics are rapidly deteriorated. That is, there is no material suitable as a photovoltaic material of a wide optical gap having a sensitivity in a short wavelength region, and there is no transparent solar cell of a high efficiency and a wide area.
On the other hand, a see-through type solar cell, through which the opposite side can be seen, is proposed, a large proportion of which is one having many minute holes therein. It is as if the opposite side is seen through a sieve, and cannot be applied to a display device, which is minute and should exhibit a sufficient brightness with a low electric power, or a window of an automobile, which requires both lighting and safety.
In recent years, a nitride series compound semiconductor receives attention as a semiconductor material of a wide optical gap. However, because such a semiconductor represented by GaN is produced through crystal growth at a temperature of from 800 to 1,000.degree. C., and sapphire is frequently used as a substrate, it is difficult to apply to a solar cell from the standpoint of cost and size.